Senator Harry Reid, the majority leader, was hospitalized Friday after falling ill in the morning, his office said. Mr. Reid, who just turned 74, was going to remain in the hospital as a precaution. But Mr. Reids spokesman, Adam Jentleson, said the senators condition had improved. However, because of his illness, Reid will not be presiding over the Senates final day of business this year. Senators will return in early January.